Process of developing electrostatic images



Oct. 24, 1961 E. c. OLSON 3,005,726

PROCESS OF DEVELOPING ELECTROSTATIC IMAGES Filed May 1, 1958 INVENTOR. Everette C. Olson United States Patent() 3,005,726 PROCESS OF DEVELOPING ELECTROSTATIC IMAGES Everette C. Olson, Columbus, Ohio, assignor, by mesne assignments, to Xerox Corporation, a corporation of New York Filed May 1, 1953, Ser. No. 732,397 4 Claims. (Cl. 117-37) This invention relates in general to xerography and in particular to a method for the development of electrostatic images.

In xerography it is usual to form an electrostatic image on a surface. One method of doing this is to charge a photoconductive insulating surface and then dissipate the charge selectively by exposure to a pattern of activating radiation. The basic xerographic process is described in U.S. 2,297,691 to C. F. Carlson. Other means of forming electrostatic images are set forth in US. 2,647,464 to James P. Ebert. Whether formed by these means or any other, the resulting elestrostatic charge pattern is conventionally utilized by the deposition of an electroscopic material thereon through electrostatic attraction whereby there is formed a visible image of electroscopic particles corresponding to the electrostatic image. Alternatively the electrostatic charge pattern may be transferred to an insulating film and the electroscopic particles deposited thereon to form the visible image. In any case this visible image in turn may be transferred to a second surface to form a xercgraphic print. The process of depositing the electroscopic powder on the electrostatic image to render the electrostatic image visible is called the development step and is one of the most critical steps of the entire process.

Now, in accordance with the present invention, a method is provided for substantially improved development of an electrostatic image. This process is distinguished by greatly reduced development times and by its unexcelled flexibility of operation with a wide range of electroscopic materials.

The figure is a diagrammatic side elevation in crosssection of apparatus for carrying out the process of the instant invention.

It is evident that no picture can be better than its development step permits. About the coarsest type of image reproduced by a xerographic process requires a resolution of at least about 50 lines per inch. Commercial xerographic line-copying machines generally have a resolving power of about 125 to 250 lines per inch. The process used in obtaining this resolution is set forth in US. 2,618,552 to E. N. Wise and involves the use of a finely-divided colored material called a tner" deposited on a more coarsely divided material called a carrier. This two-component developer is cascaded across the electrostatic image areas. When applied to continuous tone development where resolutions of about 1200 or more lines per inch are often desired, it has been found impossible to obtain this high quality of reproduction using such a system. Accordingly, a system is preferred in which an air or gas suspension of finely-divided colored material is brought near the electrostatic image in order to deposit the material on the image. This process is termed powder cloud development. The powder cloud may be generated either from a dry powder material or by forming a spray of liquid droplets to produce an air or gas suspension. The cloud so produced is then charged. As originally developed in US. 2,725,304 to R. B. Landrigan et a1. and in U.S. 2,784,109 to L. E. Walkup, charging was carried out by means of a corona spray in the case of the dry powder and by induction in the case of the liquid spray. The charged powder cloud Patented Oct. 24, 1961 was then introduced into a confined space or settling chamber wherein it contacted the surface bearing the electrostatic image. I

In order to obtain the degree of tonal rendition desired, it is necessary to draw the lines of force of the electrostatic image externally above the image-bearing surface. This is done by positioning a conductive electrode, termed a development electrode, closely adjacent to the surface bearing the electrostatic image. To be effective, the spacing between the development electrode and the image-bearing surface is very narrow, generally being no more than about -inch. The time necessary for the electrostatic particles to circulate through this restricted spacing in order to obtain adequate image densitiesresults in lengthy development times. While under some circumstances development was substantially accomplished in about 20 seconds, as a general rule development required several minutes. In addition, as described in Example 1 of the aforementioned Walkup patent, the DC potential applied in induction charging was generally of the order of 1,000 volts thus requiring a high voltage power supply. The corona generating potentials required by Landrigan et a1. were of the order of 6,500 volts or higher.

As a result, to obtain faster development times and to simplify the charging mechanism, devices such as those described in US. 2,815,734 to C. F. Carlson were developed. In such devices the development electrode is stationed about fifteen thousandths of an inch away from the surface bearing the electrostatic charge and the powder cloud passed through the development space created between these elements. In order to prevent deposition of developer powder in unwanted areas such as a background area, it is necessary to use high flow rates of the powder cloud through the development zone so that the air or gas conveying the powder acts to prevent such unwanted deposition by a self scavenging action. Charging is accomplished by blowing the powder cloud through a restricted nozzle in turbulent flow whereby the powder is charged by triboelectric contact with the walls of the charging nozzle. With the fast flow rates inherent in this process, development can be regularly accomplished in a matter of 2 or 3 seconds.

While excellent quality images can be thus produced in extremely short times, the method also has some drawbacks. Thus, the flow of the suspension of powder in air through the development zone is highly critical. Great care and ingenuity are required in designing mechanisms conveying the powder cloud to and through the development zone. Without elaborate precautions in this design a number of imperfections are introduced into the powder images. Further, the properties of the powder which is used in this process are highly critical. Very few materials have the correct combination of xerographic properties to make them operable in this process. As a result, for all practical purposes, development in this process is generally restricted to finely-divided wood charcoal.

If it is desired to develop a continuous tone image with other developer materials than wood charcoal, it is still necessary to use the devices generally disclosed by Walkup and Landrigan et al. This is particularly so where it is desired to develop a continuous tone color reproduction of an electrostatic image. In such a process the desired dye necessary to obtain the correct color is dissolved in a suitable solvent and sprayed through an atomizer to obtain the desired particle size and charged by induction as described in Walkup. The thus generated suspension of dye in air or gas is then contacted with the surface bearing the electrostatic image as shown in FIG. 10 of Walkup. Using the device of FIG. 1 of Landrigan et a1.

wherein the powder cloud passes through the development zone to reach the exit from the chamber 13 faster development times are possible. For high quality development, development times of several minutes are essential. It has now been found that one ofthe prime reasons for the: extended development times required in this process is the space charge caused by induction-charging. In thepowder cloud development process of U.S. 2,815,- 734 the large volumes of airused convey the powder cloud through the development system under such. Strong forcesthat space charge effects are negligible. Spattering of liquid droplets throughout the system prevents the use of a similar system with a liquid spray thusrequirin-g the much lower air velocities used in either Landrigan et al. 'or Walkup. Under these conditions space charge effects in the development system have been found to be critically important. Induction charging of the liquid droplets is highly efiicient resulting in almost unipolar clouds, -i.e., almost all particles have the same polarity of charge. The space charge resulting from this causes large proportions of the cloud to settle out on the walls of the apparatus so that only a very small amount of charged particles reach the development zone.

It has now been found that if tan A.C. potential is applied to the induction charging ring charging the liquid droplets as in the process of Walkup or Landrigan et al.,

that the quantity of particles bearing the desired polarity tials thereby considerably simplifying the charging apparatus.

The general nature of the process of the invention having been set forth, a specific embodiment of the invention will now be described in connection with the attached figure. The invention is not limited to this embodiment which is presented merely for purposes of illustrating means of working the instant invention.

' As shown in the figure the apparatus illustrated contemplates generally a cloud generating unit, a cloud charging unit and a development box. The cloud generating unit comprises a liquid 19 which is placed in the reservoir 18 of the air brush 20 and sprayed by compressed air fed through hose 17 into a space 10 in which a plate '11 hearing an electrostatic image thereon is suspended face downward (i.e., the face bearing the electrostatic image'is downward), over a development elec- Mode 13 thereby forming a development zone therebetween. Air brush 20 may be of conventional construction to provide a liquid feed and an air jet for atomizing the liquid and projecting it as a fine spray or mist. In order to apply a charge to the liquid particles as they are sprayed, a conductive ring electrode 15 is supported around the tip of the air brush so that the jet of liquid droplets is projected out through the ring- Ring 15 is insulated from the air brush 20 by insulating spacer 16 but is connected by an insulated conductor 21 to a source of A.C. potential 22. The conductive parts of the air brush are connected to the other (ground) terminal of potential source 22. This provides an electrostatic field facing down toward development electrode 13 thereby forming a development zone therebetween. Adjacent to one end of this development electrode 13 is inlet slit 24 connecting space 10 in box 23 with the development zone, while adjacent the other end of electrode 13 is exit slit 14. The liquid spray entering space 10 from air brush 20 through charging ring 15 is forced by the slightly'increased pressure through the development zone between the development electrode 13 and xerographic plate 11 and out exit means 14. In passing through the development zone, electroscopic particles are attracted from the air or gas stream and deposit on the plate in accordance with the electrostatic image thereon.

To illustrate the advantages of the instant invention the apparatus illustrated was used with a conductive plate substituted for the xerographic plate 11 and auniform potential of 540 volts maintained between the development electrode 13 and the metal plate 11 to simulate a uniform electrostatic image. Reservoir 18 was charged with a' supply of liquid 19 consisting of cyan dye dissolved in ethyl alcohol. The nozzle of air brush 20 was adjusted to spray the liquid 19 at the rate of approximately 60 cc. per minute to produce particles the sizerange of less than 25' microns In the first run a sourceof D.C. potential was substituted for the source of A.C. potential 22 so that +500 volts D.C. were applied toring 15. Development proceeded for 60 seconds. The powder deposited on both the development electrode and the metal plate substituting for the xerographic plate were then separately transferred using slightly dampened dye transfer paper and the optical density of the deposits measured. It was found that the plate having the relative positive polarity had an optical density of 0.16, while the density on the plate having the relative negative charge was 0.05.

The run was then repeated using exactly the same conditions, that is, the same apparatus using the same liquid 19, the same rate through the nozzle, the same potential between plate 11 and electrode 13, and the same development time. However, an A.C. source 22 was substituted 'for'the D.C. source to place a charge of 20 volts A.C.

on induction charging ring 15. The deposits on each plate were again transferred to slightly dampened dye transfer paper and the optical density of the deposits measured. It was found that the density on the positive plate had increased over three-fold to 0.55, while the density on the negative plate had increased almost ninefold to 0.44.

The distance between the development electrode and the image-bearing surface can be no more than about As-inch to effectively draw the field of force of the electrostatic image externally above the image-bearing surface and thus is particularly critical. For good photographic quality the distance between the development electrode and the image-bearing surface should be no more than about A -inch. The development electrode may present a continuous conductive surface to the xerographic plate in which case the eleetroscopic particles are flowed through the development zone parallel to the surfaces of the xerographic plate and the development electrode. Alternatively, the development electrode may present a foraminous surface to the xerographic plate (as disclosed in US. 2,784,109 to Walkup) in which case between the liquid outlet of the air brush 20 and ring 15 so that liquid particles leaving the brush acquire electrostatic charges by induction as they are ejected.

' Development of the electrostatic image is effected by blowing the liquidv mist into the confined space 10. formed by container 23. A xerographic plate 1 1 bearing an elec: trostatic image thereon is placed on shoulder 12 of container '23 with the race bearing the electrostatic image velopment of the electrostatic image. In reversal (nega tive) development, rather than grounding the development electrode a potential is applied thereto equal in polarity and magnitude to the highest charge appearing on the xerographi-c plate. As a result the charges on the surface of the xerographic plate are effectively reversed to thereby present a negative or reversal image. Using lower electrostatic potentials on the development electrode combined with multiple development steps, it is possible to vary the tonal quality of the resulting developed image over a wide range as is more fully described in the above-mentioned Walkup patent.

The standard commercial plate used in xerography today comprises vitreous selenium on an aluminum or brass substrate. By reason of the unique electrical properties of selenium, such a plate is normally used only with positive sensitization. Other xerographic plates such as an alloy of arsenic with selenium (as described in U.S. 2,803,542) or certain binder plates, that is, photoconcluctive insulating pigments dispersed in an insulating resin binder and coated on a conductive backing, are often used with negative sensitization. The process of the instant invention, affording an almost equal quantity of positively and negatively charged electroscopic particles, is useful in the development of electrostatic images with either polarity of charge on any xerographic member.

The liquid used as the suspending or dissolving agent for the electroscopic particles in the air brush should be at least slightly conductive if the induction charging process is to be efficiently carried out. The electrical conductivity of the liquid to be atomized will also depend, to some extent, on the nature of the coloring agent dissolved and/or dispersed therein. Thus, without specifying whether the requisite conductivity is imparted by ions dispersed in the liquid or is an intrinsic property of the liquid itself, this necessary property will be described as electrically conductive liquid containing marking material.

The electroscopic particles contacted with the electrostatic image-bearing surface may be either liquid or dry powder, that is, the means provided to convey the cloud to the surface bearing the electrostatic image after the cloud is charged may be so designed that the particles in the cloud or aerosol are still in liquid form when contacted therewith or it may be so designed that the liquid will evaporate from the droplets so that only dry powder reaches the image-bearing surface. It has been found that on evaporation of the liquid from a droplet which has been inductively charged as described herein, that the electrostatic charge remains on the dry residue from the droplet.

Induction charging, as described herein, does not rely on any critical triboelectric relationship. Any material which can be either dissolved or dispersed in a slightly conductive liquid may be efiiciently charged. The atomization process permits ready control of the size and concentration of the particles in the cloud, while varying the potential on the induction ring controls the amount of charge per particle. Thus, the instant process provides a means for creating a powder cloud of either liquid or dry particles of virtually any material which the operator desires to use to develop electrostatic images.

I claim:

1. The process for developing an electrostatic image on an insulating surface wherein a cloud of electroscopic material is passed between the image-bearing surface and a conductive development electrode, said process comprising placing a conductive electrode in closely-spaced parallel relation to an electrostatic image-bearing surface thereby drawing the lines of force of said electrostatic image externally above said surface, forming a cloud of marking materials having substantially zero net charge and comprising a mixture of suspended particles of opposite electrical polarity by atomizing an electrically conductive liquid containing marking material from a nozzle and applying an A.C. potential adjacent to said nozzle to induce substantially equal quantities of opposite polarity charges on various cloud particles and passing said charged particles between the image-bearing surface and the conductive development electrode whereby said charged particles deposit on said surface in conformity with said electrostatic image.

2. The process for developing an electrostatic image on an insulating surface wherein a cloud of electroscopic material is passed between the image-bearing surface and a conductive development electrode, said process comprising placing a foraminous conductive electrode in closely-spaced parallel relation to an electrostatic imagebearing surface thereby drawing the lines of force of said electrostatic image externally above said surface, forming a cloud of marking materials having substantially zero net charge and comprising a mixture of suspended particles of opposite electrical polarity by atomizing an electrically conductive liquid containing marking material from a nozzle and applying an A.C. potential adjacent to said nozzle to induce substantially equal quantities of opposite polarity charges on various cloud particles and passing said charged particles through said foraminous electrode within the lines of force of said electrostatic image whereby said charged particles deposit on said surface in conformity with said electrostatic image.

3. The process for developing an electrostatic image on an insulating surface wherein a cloud of electroscopic material is passed between the image-bearing surface and a conductive development electrode, said process comprising spacing a conductive electrode no more than about /8 from an electrostatic image-bearing surface, forming a cloud of marking materials having substantially zero net charge and comprising a mixture of suspended particles of opposite electric polarity by atomizing an electrically conductive liquid-containing marking mate rial from a nozzle and applying an A.C. potential adjacent to said nozzle to induce substantially equal quantities of opposite polarity charges on various cloud particles, and passing said charged particles between the image-bearing surface and the conductive development electrode, whereby said charged particles deposit on said surface in conformity with said electrostatic image.

4. The process for developing an electrostatic image on an insulating surface wherein a cloud of electroscopic material is passed between the image-bearing surface and a conductive development electrode, said process comprising spacing a conductive foraminous electrode no more than about 4; from an electrostatic image-bearing surface, forming a cloud of marking materials having substantially Zero net charge and comprising a mixture of suspended particles of opposite electric polarity by atomizing an electrically conductive liquid-containing marking material from a nozzle and applying an A.C. potential adjacent to said nozzle to induce substantially equal quantities of opposite polarity charges on various cloud particles and passing said charged particles through said forarninous electrode, whereby said charged particles deposit on said surface in conformity with said electrostatic image.

References Cited in the file of this patent UNITED STATES PATENTS 1,911,808 Collins May 30, 1933 1,958,406 Darrah May 15, 1934 2,457,256 Melton et al. Dec. 28, 1948 2,638,416 Walkup May 12, 1953 2,725,304 Landrigan et al Nov. 29, 1955 2,784,109 Walkup Mar. 5, 1957 2,752,833 Jacob July 3, 1957 2,824,813 Fauser et al. Feb. 25, 1958 2,861,543 Crumrine et al. Nov. 25, 1958 2,880,699 Hayford Apr. 7, 1959 

1. THE PROCESS FOR DEVELOPING AN ELECTROSTATIC IMAGE ON AN INSULATING SURFACE WHEREIN A CLOUD OF ELECTROSCOPIC MATERIAL IS PASSED BETWEEN THE IMAGE-BEARING SURFACE AND A CONDUCTIVE DEVELOPMENT ELECTRODE, SAID PROCESS COMPRISING PLACING A CONDUCTIVE ELECTRODE IN CLOSELY-SPACED PARALLEL RELATION TO AN ELECTROSTATIC IMAGE-BEARING SURFACE THEREBY DRAWING THE LINES OF FORCE OF SAID ELECTROSTATIC IMAGE EXTERNALLY ABOVE SAID SURFACE, FORMING A CLOUD OF MARKING MATERIALS HAVING SUBSTANTIALLY ZERO NET CHARGE AND COMPRISING A MIXTURE OF SUSPENDED PARTICLES OF OPPOSITE ELECTRICAL POLARITY BY ATOMIZING AN ELECTRICALLY CONDUCTIVE LIQUID CONTAINING MARKING MATERIAL FROM A NOZZLE AND APPLYING AN A.C. POTENTIAL ADJACENT TO SAID NOZZLE TO INDUCE SUBSTANTIALLY EQUAL QUANTITIES OF OPPOSITE 